Annual two-dose tetanus toxoid vaccination induces protective humoral immunity to all age groups of rhesus macaques.

A tetanus outbreak occurred during 2014-2015 in the rhesus macaques reared in an open enclosure in our facility. As the soil of the facility was suspected to be contaminated with Clostridium tetani spores, there was a risk of further tetanus occurring among the macaques. To protect them from tetanus, a tetanus toxoid vaccination was recommended; however, the vaccinated elderly animals might not be effectively protected due to insufficient humoral immune responses. Hence, we evaluated the dynamics of antibody responses among rhesus macaques of all age groups vaccinated with two-dose tetanus toxoid at a 1-year interval during a 3-year follow-up study. The vaccination developed anti-tetanus toxin-specific antibodies in animals of all age groups, the antibody levels peaked 1 year after the second vaccination, and the peak levels decreased with age. However, the levels among elderly individuals (aged ≥13 years) were still higher than the threshold level, which was supposed to protect them from tetanus development. Although the rhesus macaques in our facility had a risk of occasional exposure to the spores due to the outbreak, no incidence of tetanus has ever occurred to date. These results indicate that the vaccination protocol is effective in protecting not only younger but also older animals from tetanus.